1. Field of the Invention
The present invention relates to a solid-state image sensing device and, more particularly, to a solid-state image sensing device including an analog to digital converting device.
2. Description of the Related Art
Some solid-state image sensing devices include a column parallel type analog to digital converting device (to be simply referred to as an “ADC” hereinafter) arranged at the output of a vertical signal line. Examples of the ADC are a counter ramp ADC that uses single slope architecture, and a successive approximation ADC.
Japanese Patent Laid-Open No. 2007-243324 (literature 1) describes a technique of using two ADCs to convert an analog signal into an n-bit digital signal. That is, the first ADC performs analog to digital conversion (to be referred to as “AD conversion” hereinafter) for the m most significant bits, and performs digital to analog conversion (to be referred to as “DA conversion” hereinafter) for an m-bit digital signal. An analog signal obtained by subtracting a signal value as the result of the DA conversion is input to the second ADC, which then performs AD conversion for the n-m least significant bits.
That is, the technique described in literature 1 shortens the time required to perform AD conversion for n bits by separating it into two processes in which the first ADC performs AD conversion for a bit length of m bits and the second ADC performs AD conversion for a bit length of n-m bits.
Furthermore, the technique described in literature 1 samples a voltage corresponding to the bit length of n-m bits of the second ADC by charging a capacitor, and executes gain processing for the sampled voltage.
In the second ADC of literature 1, a constant-current source is arranged for each column. As the current of the constant-current source varies, the amount of electric charge of the capacitor also varies, thereby causing a conversion error. That is, a variation in current of the constant-current source in literature 1 causes a different conversion error for each column, which is reproduced as different vertical noise for each column. In other words, a variation in current of the constant-current source deteriorates the image quality of an image output by a solid-state image sensing device.